ECHINODERM ATA — CRINOIDS 1 6 1 



at its lower end. The stalks are very long, attaining at times 

 a length of at least twenty feet. 



The calyx is made up dorsally of calcareous plates definitely 

 arranged ; the plates upon the ventral surface are without 

 definite order. All the plates are firmly united, forming a rigid 

 skeleton. The mouth occupies the center of the upper surface 

 and from it radiate five open amxbulacral or food grooves to the 

 tips of the arms. These arms, unlike the usually simple ones 

 of the starfish, branch repeatedly, each branch in turn giving 

 off many short side branches or pinnules. The ambulacral 

 grooves branch correspondingly. These grooves are lined 

 with cilia by the aid of which the food, chiefly diatoms, pro- 

 tozoons, and microscopic crustaceans, is urged into the mouth. 

 The ambulacral system here, as in the starfish, forms a ring 

 around the mouth and sends a ray to each pinnule ; connected 

 with this ring are several ciliated, branched water-tubes opening 

 into the coelome. Into this coelome, the body cavity, water 

 enters from the exterior by means of minute water-pores through 

 plates on the ventral surface. Small, distensible tube-feet, 

 the tentacles, are present ; they, however, lack suckers, and are 

 not locomotor but merely tactile and respiratory in function. 

 The water-pores and water-tubes represent the madreporite 

 and its canal of the starfish. 



The mouth opens through an oesophagus into a wide stomach, 

 thence into the intestine, which coils inside the body wall, but 

 does not have extensions into the arms. It opens by the anus 

 through a tube eccentrically placed upon the upper (ventral) 

 surface. The blood vascular system consists of a ring around 

 the oral end of the digestive canal, giving off radial vessels to 

 the arms and the stalk. 



The nervous system consists of (i) the ambulacral portion, — 

 a ring around the mouth from w^hich pass series of nerves to arms 

 and pinnules through the base of the ambulacral grooves ; and 

 (2) the axial portion, — a five-angled organ beneath the basal 

 plates at the junction of calyx and stalk, from which radiate 



